1. Field of the Invention
The present invention relates to a charged particle optical apparatus, and more particularly to a SEM with lowered electron-electron interaction.
2. Description of Related Art
Defect inspection of semiconductor wafers and masks for IC manufacturing is an accepted production process for yield enhancement. The information obtained from a wafer defect inspection tool can be used to flag defective dies for repair, or improve wafer processing parameters. The systems used for inspection were typically optical microscopy in nature. However, for imaging of nanoscale objects, optical microscopy has limited resolution since the objects are often much smaller than the wavelength of measuring light. Electron beam systems can have much higher resolution than optical systems because the wavelength of the electron can be in the angstrom regime.
Imaging of an electron beam system are usually carried out using a tightly focused beam (electron probe), the spatial resolution of the system refers to the smallest beam diameter from which spatial information can be obtained. The probe size in general is determined by the electron source size, lens aberrations and electron diffraction at the condenser aperture; however, it can in principle be broadened by Coulomb interactions between the electrons within the electron beam illumination system.
The image resolution of an electron beam system is subject to degradation by lens aberrations. Usually chromatic aberration is the most important factor, since the energy and angular widths of the focused electrons are considerable. Adopting a monochromator or an energy filtered unit to the electron beam system can decrease energy spread of the electron beam and thereafter reduce the chromatic aberration of the system.
However, how to decrease Coulomb interactions between electrons is still an important issue in electron beam system. A conventional solution to this issue is to increase primary beam energy, but landing energy is also increased. If a higher landing energy is not preferred to the practice, a larger retarding force must be applied to the focused primary beam between the objective lens and the specimen, which will incur much more complicated mechanism to the electron beam system. Therefore, the present invention discloses an apparatus to reduce the Coulomb interactions between the electrons and improves the resolution of the electron beam system with a simple and more reliable design.